LT1573CS8-2.5#PBF Datasheet LT1573CS8-2.5#PBF, LT1573CS8#PBF, LT1573IS8#PBF | P7-P9

LT1573

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Figure 2. Basic Regulator Circuit

on the regulator circuit under overload conditions. The

resistor R

is chosen based on the operating requirements

of the circuit, primarily the dropout voltage and the output

current. The dropout voltage of an LT1573-based regula-

tor circuit is determined by the V

saturation voltage of

the discrete external PNP transistor when it is driven with

a base current equal to the available drive current of the

LT1573.

External PNP Transistor Selection Criteria

The selection of an appropriate external PNP transistor

depends on the regulator application specifications. The

critical PNP transistor selection criteria include:

1. The maximum output current of the PNP transistor

2. The dropout voltage at the maximum output current

3. The gain-bandwidth product f

of the transistor

The PNP transistor must be able to supply the specified

maximum regulator output current to be qualified for the

regulator application. The V

saturation voltage of the

transistor at the maximum output current determines the

dropout voltage of the circuit. The dropout voltage deter-

mines the minimum regulator input voltage for a certain

specified output voltage. The gain-bandwidth product f

of the transistor determines how fast the voltage regulator

can follow an output load change without losing voltage

regulation.

The D45H11 from Motorola and the KSE45H11TU from

Samsung can be used in all LT1573 regulator circuits with

current ratings up to 5A. The D45H11 can supply 5A of

COMP

OUT

DRIVE

LATCH

SHDN

GND

LT1573

TIME

OUT2

OUT1

OUT

GND

1573 F02

LOAD

+ +

OUT

The LT1573 is designed to be used in conjunction with an

external PNP transistor. The overall specifications of a

regulator circuit using the LT1573 and an external PNP will

be heavily dependent on the specifications of the external

PNP. While there are a wide variety of PNP transistors

available that can be used with the LT1573, the specifica-

tions given in a typical transistor data sheet are of little use

in determining overall circuit performance. In the follow-

ing discussion the critical requirements of the PNP tran-

sistors are noted. Design equations are given and

examples are shown using a readily available discrete PNP

transistor. This device is inexpensive, available from mul-

tiple sources and can be used for a wide range of applica-

tions. For applications using other PNP transistors, the

regulator specifications can be derived by the same method.

Basic Regulator Circuit

The basic regulator circuit is shown in Figure 2. The

adjustable output LT1573 senses the regulator output

voltage from its feedback pin via the output voltage

divider, R1 and R2, and drives the base of the external PNP

transistor to maintain the regulator output at the desired

value. For fixed output versions of the LT1573, the regu-

lator output voltage is sensed from the feedback pin via an

internal voltage divider. The resistor R

is required for the

overcurrent latch-off function. R

is also used to limit the

drive current available to the external PNP transistor and

to limit the power dissipation in the LT1573. Limiting the

drive current to the external PNP transistor will limit the

output current of the regulator which minimizes the stress

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Current Limit

For regulator circuits using the LT1573, current limiting is

achieved by limiting the base drive current to the external

PNP pass transistor. This means that the actual system

current limit will be a function of both the current limit of

the LT1573 and the Beta of the external PNP. Motorola

provides the following Beta information for the D45H11.

The minimum Beta of the D45H11 is 60 when V

= 1V and

= 2A. The minimum Beta is 40 when V

= 1V and I

4A. For other PNP transistors, the user should first find out

the Beta information from the external PNP transistor

manufacturer to determine the appropriate LT1573 base

drive current limit. The current limit of the regulator

system then can be achieved by selecting the appropriate

amount of resistance R

in Figure 2.

Selecting R

Resistor R

can be used to limit the available drive current

to the external PNP transistor. In order to select R

, the

user should first choose the value of the drive current that

will give the required value of output current and dropout

voltage. For a circuit using the D45H11 as a pass transistor

this can be done using Table 1. For circuits using transis-

tors other than D45H11, the user must characterize the

transistor to determine the drive current requirements for

the specified output current and dropout voltage. In gen-

eral, it is recommended that the user choose the lowest

value of drive current that will satisfy the output current

requirements. This will minimize the stress on circuit

components during overload conditions.

The formula used to determine the resistor R

is:

= (V

– V

DRIVE

)/(I

DRIVE

+ I

) (1)

where,

= the minimum input voltage to the circuit

= the maximum emitter/base voltage of the PNP

pass transistor

DRIVE

= the minimum PNP base current required

= the current through R

= V

DRIVE

= the DRIVE pin saturation voltage when the DRIVE

pin current equals (I

DRIVE

+ I

)

output current with dropout voltage as low as 0.35V. The

gain-bandwidth product f

of the D45H11 is typically

40MHz which enables the regulator, composed of this

PNP transistor and the LT1573, to handle the load changes

of several amps in a few hundred nanoseconds with a

minimum amount of output capacitance.

The following sections describe how specifications can be

determined for the basic regulator based on the LT1573

and D45H11 from Motorola. To determine the specifica-

tions for regulators formed by the LT1573 and other PNP

transistors, a similar method can be used.

Dropout Voltage

The dropout voltage of an LT1573-based regulator circuit

is determined by the V

saturation voltage of the discrete

external PNP transistor when it is driven with a base

current equal to the available drive current of the LT1573.

The LT1573 is guaranteed to sink 250mA of base current

(440mA typ). The available drive current of the LT1573 can

be reduced by adding a resistor (R

in Figure 2) in series

with the DRIVE pin. Table 1 lists some useful operating

points for the D45H11. These points were empirically

determined using a sampling of devices.

Table 1. D45H11 Dropout Voltage

TYPICAL

DRIVE CURRENT OUTPUT CURRENT DROPOUT VOLTAGE

(mA) (A) (V)

20 1 0.20

20 2 0.50

40 2 0.25

40 3 0.50

60 3 0.25

60 4 0.70

80 4 0.45

100 4 0.35

100 5 0.70

150 5 0.40

200 5 0.35

150 6 0.65

200 6 0.45

250 7 0.50

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Resistor R

helps to turn off the PNP (Q

OUT

in Figure 2).

Smaller values for R

turn off the PNP faster but will

increase input current. The recommended value for R

50Ω. For circuits that do not require high output current or

fast transient response, the value of R

can be increased

up to 200Ω. For the D45H11, the emitter-base voltage is

a function of base and collector current. Table 2 lists some

useful operating points for the D45H11. These points were

empirically determined using a sampling of devices.

Table 2. D45H11 V

AT 25

(mA) (A) (V)

1 0.2 0.65

7 1 0.75

23 2 0.80

45 3 0.85

66 4 0.90

100 5 0.95

Design Example

Given the following operating requirements:

4.5V < V

< 5.5V

OUT(MAX)

= 5A

OUT

= 3.3V

1. The first step is to determine the required drive current

for the D45H11. Dropout voltage must be less than 1.2V

at 5A output current. From Table 1, a drive current of

100mA will give 0.7V dropout voltage at an output

current of 5A. This satisfies the operating require-

ments.

2. The next step is to determine the value of R

. Assume

is 50Ω. From Table 2, the maximum emitter-base

voltage for this design is 0.95V. The current through

is:

= V

= 0.95/50 = 19mA

DRIVE

is the DRIVE pin saturation voltage when the

DRIVE pin current equals 119mA, which can be read

from the typical performance characteristics curve to

be 0.39V. Resistor R

now can be calculated from

Eq (1):

= (4.5 – 0.95 – 0.39)V/(100 + 19)mA = 26.6Ω

The next lowest 5% value is 24Ω.

Overcurrent Latch-Off

In addition to limiting the base drive current, the resistor

is included in the circuit for the overcurrent protection

latch-off function. There is a minimum value for this

resistance. It is calculated by Equation 1 with the drive

current I

DRIVE

set to the minimum available drive current

(= 250mA) from the LT1573. At high currents, R

also

limits the power dissipation in the LT1573. In some

conditions, resistor R

can be replaced with a short. This

is possible in circuits where an overload is unlikely and the

input voltage and drive requirements are low. If resistor R

is not included in the circuit, the regulator is protected

against the overcurrent condition only by the thermal

shutdown function. After the resistor R

is determined, a

certain amount of base drive current is available to the

external PNP transistor. An overcurrent or output short

condition will demand a base drive current greater than the

LT1573 can supply. The internal drive transistor will

saturate. A time-out latch will be triggered by this

overcurrent condition to turn off the regulator system. The

time-out period is determined by an external capacitor

connected between the LATCH and GND pins. The time-

out period is equal to the time it takes for the capacitor to

charge from 0V to the latch threshold which is equal to

. The latch charging current is set by an internal

current source and is a function of input voltage and

temperature as shown in the typical performance charac-

teristics curve. At 25°C, the typical latch charging current

ranges from 7.2µA with 3V input to 8µA with 7V input. If

the overcurrent or output short condition persists longer

than the time-out period, the regulator will be shut down.

Otherwise, the regulator will function normally. In the

latch-off mode, some extra current is drawn from the input

to maintain the latch. The latching current is a function of

input voltage and temperature as shown in the typical

performance characteristic curve. At 25°C, the typical

latching current ranges from 0.3mA with 3V input to

9.5mA with 7V input. The latch can be reset by recycling

input power, by grounding the LATCH pin or by putting the

device into shutdown.

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LDO Voltage Regulators Low Dropout Reg Driver 2.5V

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